We paid $2.00 per litre at Mt Dare on the 4th of June this year, and $1.60 per litre when we got to Birdsville.
I do believe that the price went up just after we filled at Mt Dare though.
When I went to pay, the bloke at the till asked the woman that unlocked the bowser what the price was, and she said, "It's still set to $2.00, but I'm just about to put it up." So it could be completely different by now.
EDIT: And we paid $1.279 at Coober Pedy on the 10th of May.